Printed circuit boards (PCB) are the common denominator in virtually all electronics products and the circuit board manufacturing industry is huge. It is often seen as "low tech", because the manufacture of circuit boards has generally been accomplished with rather simple tools; but, with the growing push towards miniaturization, line widths and pad sizes are decreasing, with the result that current manufacturing techniques are struggling to maintain alignment between layers in a circuit board. The problem arises with the fact that circuit boards are made from many thin layers which each have unique circuit patterns, and which must align so that interconnection holes (passages) can be formed between them. These thin sheets are like large pieces of paper and are typically large fiberglass sheets coated with copper which is then covered with photoresist, exposed, and then etched. They are exceedingly difficult to position repeatably. Typically, overconstrained arrangements of slots and holes are used to fit over close tolerance posts. Currently, the industry standard is to use precision-punched holes in sheets and precision alignment posts, which, due to tooling errors and environmental issues, result in positioning errors on the order of hundreds of microns. For very thin flexible sheets conventional fixturing methods are not applicable. The first problem of tooling errors-tolerances between the slot size and the post size, and errors in their relative positions and orientations; and the second problem of post wear residing in the use of fiberglass which is super abrasive, result in a practical positioning repeatability of only about 100 to 200 microns. Furthermore, dimensions of punched holes and slots vary with material thickness due to the mechanics of the glass-epoxy material that forms the very struicture of the circuit board.
Other industries, such as the aircraft industry, also have similar positioning problems with large material sheets. These same issues apply, although on a larger scale. The fixturing method that is the subject of this invention is equally applicable to such and similar applications, as well, and is being described illustratively for the example of printed circuit boards.
A key to the design philosophy that guides the use of this invention in all its different applications is the recognition that the loading is succinctly different from that in a traditional kinematic coupling. Accordingly, it is appropriate to review the mathematical background to the technology--an area called screw theory. Screw theory asserts that the motion of any system can be represented by a combination of a finite number of screws of varying pitch that are connected in a particular manner. This concept is well illustrated for a plethora of mechanisms by Phillips (J. Phillips, Freedom in Machinery, Vol. I, Cambridge University Press, London, 1982, p 90.). Earlier work on screws spanned the latter half of the 19th century, and a detailed summary of such work on screw theory was published in 1900 (R. S. Ball, A Treatise on the Theory of Screws, Cambridge University Press, London, 1900). Ball's treatise describes the theory of screws in elegant, yet easily comprehensible linguistic and mathematical terms.
Screw theory is an elegant and powerful tool for analyzing the motion of rigid bodies in contact, but it is not always easy to apply. With respect to practical implementation of the theoretical requirement for stability, for traditional precision three-groove kinematic couplings, such as described by applicant Slocum (A. Slocum, Precision Machine Design, Prentice Hall, 1991, Section 7.7), stability, and good overall stiffness will be obtained if the normals to the plane of the contact force vectors bisect the angles of the triangle formed by the hemispheres (e.g., balls) that lie in the grooves.
This works well for kinematic coupling of one rigid body to the next. When one wants to use a kinematic coupling to define the position and orientation (yaw) of a body in a plane, and then have the body translate down and contact a plane, however, one needs to use a translational kinematic coupling as described in applicant's co-pending U.S. patent application Ser. No. 568,612 (Dec. 7, 1995).
For thinner sections, it is possible to cut grooves in the body, if the grooves can still be made to have sufficient rigidity to support contact on angled force lines. This works for applications such as silicon wafers as disclosed in, for example, applicant's co-pending U.S. patent application Ser. No. 324,255 (Oct. 17, 1994). Current state of the art, however, uses non-kinematic solutions involving use of a post that fits tightly in a hole to form a pivot point, and then a post that fits tightly in a groove to form a reaction point, where the tight fit in the hole of the first pin is supposed to provide planar registration. The problem is that these accurate fits are very hard to realize in practice with great accuracy and repeatability.
There remains, however, a powerful need for means to align precisely different layers in a printed circuit board lay-up prior to heating and pressing the layers to bond them together, as well as at every step in the process that creates the layers in the first place. The issue is that the sheets that make up the PCB layers are very thin and floppy. They are typically made of fiberglass sheets on the order of a fraction of a millimeter in thickness. In addition to not being able to support any normal forces (to the plane of the sheet) it is not possible to form shaped grooves in the sheet. It is only practical to cut slots into the sheets, where the cut is straight through the sheet.
In the reprographics industry, when a large stack of paper is to have its edges aligned, such as prior to binding, the paper is put into a vibrator with walls on 2 sides at a compound angle. This shakes all the sheets down into a common corner. Such a technique is not practical for PCB sheets, however, because the sheets are so difficult to cut that sub-millimeter registration would never be achievable, though it would be possible to precision-punch registration grooves into the sheets.
Thus a special design has been needed to develop kinematic couplings specifically tailored for application to the problems of accurately positioning and orienting thin sheets of materials.